The present invention relates to a process for the automatic surface inspection of a moving strip, in particular a sheet of rolled metal, or of plates moving past at high speed, in order to detect surface defects.
A first conventional technique for the surface inspection of a strip consists in carrying out a visual check of the surface of the strip.
Such a technique has disadvantages owing to the fact that it is very constraining for the operator, and subjective, and does not ensure constancy of inspection of the strips.
In addition, it makes it necessary to cause the strip to pass via a specific line operating at reduced speed, which has a detrimental effect on the efficiency of manufacture of the strips.
Another known technique of surface inspection consists in checking the surface state of the strip automatically by means of, on the one hand, a camera whose optical axis is directed towards the surface to be inspected and which forms on the said surface a line of sight which is transverse relative to the direction of movement of the strip and, on the other hand, a lighting system illuminating the width of the strip at the location of the line of sight, and finally a signal-processing unit for the real-time analysis of the images supplied by the camera.
Conventional lighting systems of automatic surface inspection devices produce lighting whose emitted rays are, in the transverse direction, perpendicular to the strip. The result is that, at each point of the transverse line of sight, the direction of observation of the camera forms a variable angle with the specular reflected ray.
However, with this kind of lighting system, some types of defect are difficult to detect or are not detected. This is the case in particular of long mechanical defects in the direction of rolling.
In order to improve the detection of this type of defect, one solution consists in providing lighting whose rays are no longer emitted perpendicularly to the strip.
For that purpose, there are systems capable of producing this type of lighting whose principal angle of ray emission is constant in the direction perpendicular to the direction of movement of the strip.
However, the application of these systems has two major disadvantages. On the one hand, the visibility of a given defect is not constant as a function of the position of this defect along the line of sight and, on the other hand, the signal recorded by the camera and constituting the image of the strip exhibits variations in amplitude in the transverse direction.
These disadvantages are a real obstacle to efficient exploitation of the images obtained by means of such light sources.